macOS: Improve event queuing (#3708)

* Use AppKit's internal queuing mechanisms

This allows events to be queued in a more consistent order, they're now
interleaved with events that we handle immediately (like redraw events),
instead of being handled afterwards.

* Only queue events if necessary

This makes the call stack / backtraces easier to understand whenever
possible, and generally improves upon the order in which events are
delivered.

src/platform_impl/macos/event_loop.rs

@@ -252,7 +252,7 @@ impl<T> EventLoop<T> {
         });
 
         let panic_info: Rc<PanicInfo> = Default::default();
-        setup_control_flow_observers(Rc::downgrade(&panic_info));
+        setup_control_flow_observers(mtm, Rc::downgrade(&panic_info));
 
         let (sender, receiver) = mpsc::channel();
         Ok(EventLoop {